Polyhaloimides, especially tetrabromophthalimides and bis(tetrabromophthalimides), are well-known compounds which may be applied as flame-retardants to many flammable materials, particularly plastics. See, for example, S. M. Spatz et al., "Some N-substituted Tetrabromophthalimide Fire-retardant Additives", Industrial and Engineering Chemistry Product Research and Development, Vol. 8, no. 4 (1969) pps. 397-398, as well as U.S. Pat. No. 3,873,567; French Patent no. 2,369,261 and Japanese Patent Application nos. 74-045,062 and 75-064,337.
It is also well known, however, that the preparation techniques for the polyhaloimides described in the abovementioned references produce mediocre yields. In addition, they provide products which are yellow in color or which discolor when used. These products thus impart to the plastics in which they are incorporated (i.e. in the form of compounds or molded articles) an unacceptable color which rules out their use in a number of applications. Furthermore, these products very frequently contain materials which are volatile at the temperatures at which certain polymeric materials are molded or otherwise worked, which leads to corrosion of the molds used to form the finished article.
In addition, the organic solvents (particularly xylene, toluene, alcohol and acetic acid) used in these methods, which are most often chosen for their ability to form azeotropic mixtures with water (i.e., thus making it possible to entrain the water of condensation produced by the imidification reaction, or to dissolve the halogenated dicarboxylic acid anhydride), require the use of costly separation and recovery operations, as well as drying techniques suitable for the removal of the organic solvent vapors. The preceding disadvantages are most often encountered in the case of polyhaloimides derived from hydrazine and from halogenated carboxylic acid anhydrides.
What is more, the use of this latter raw material requires a prehalogenation of the dicarboxylic acid anhydrides. This factor represents an additional handicap both from the technological and economic standpoints, because the synthesis of these polyhaloimides requires the following additional sequence of steps:
halogenation of the carboxylic acid anhydride; and PA1 isolation and purification of the halogenated anhydride thus obtained. PA1 a) halogenating at least one dicarboxylic acid anhydride to obtain a crude halogenation product therefrom; and PA1 b) reacting the crude product with a sufficient amount of hydrazine to convert substantially all of the halogenated anhydride to the corresponding imide. PA1 anhydrides of aromatic (e.g., benzene, naphthalene, anthracene) dicarboxylic acids, especially those having the general formula: ##STR1## in which p is an integer ranging from 0 to 2, anhydrides of partially halogenated aromatic (e.g., benzene, naphthalene, anthracene) dicarboxylic acids, especially those having the general formula: ##STR2## in which X is a chlorine or a bromine, m varies between 0 and 4, n varies between 0 and 2, m+n is less than 4+2p, and p varies between 0 and 2. X may also be a chlorine and a bromine when m+n greater than 1. Thus, it is possible to have combinations of chlorine and bromine in the same molecule.